The field of various electronic devices using semiconductors has been and will be achieving rapid development. For the time being, silicon is used as a main semiconductor substrate. In recent years, compound semiconductors showing high-speed properties, such as GaAs, have been steadily developed.
Various semiconductor devices endowed with desired performance properties are generally obtained by forming a crystal layer having necessary characteristics on a single crystal substrate by various techniques, such as ion implantation, diffusion, and epitaxial growth. Above all, epitaxial growth has been spreading because it enables not only control of the amount of an impurity but control of the crystal composition or thickness over an extremely broad range and with high precision.
Known methods for epitaxial growth include liquid phase epitaxy, vapor phase epitaxy, and molecular beam epitaxy (hereinafter sometimes abbreviated as MBE), one of vacuum evaporation. Vapor phase epitaxy among them is capable of processing a large quantity of substrates under control and is therefore widely employed on an industrial scale. In particular, metalorganic chemical vapor deposition (hereinafter sometimes abbreviated as MOCVD), which comprises vaporizing an organometallic compound or a metal hydride of an atomic species that is to constitute an epitaxial layer and thermally decomposing the vapor on a substrate to cause crystal growth, has recently been extending its use on account of its applicability to a wide range of materials and suitability for precise control of crystal composition and thickness.
For example, a high electron mobility transistor, sometimes called a modulation doped transistor (MODFET) or a hetero-junction field effect transistor (HJFET), hereinafter inclusively referred to as HEMT, is one kind of field effect transistors (FET) and is of importance as an element of low noise amplifiers in microwave communication systems. The crystal used in HEMT is prepared by making a GaAs crystal and an AlGaAs crystal having necessary electronic characteristics successively grow in a necessary structure on a GaAs substrate by the above-mentioned vapor phase growth.
Crystals for semiconductor lasers, typical light-emitting devices, are also prepared in a similar manner, i.e., GaAs and AlGaAs layers endowed with necessary electrical characteristics, composition and thickness are made to grow on a substrate.
GaAs and an AlGaAs series are widely employed as materials for preparation of these devices since the lattice constant of the latter can be made to agree with that of the former at an arbitrary composition and can achieve various hetero-junctions while maintaining satisfactory crystal characteristics. It is also possible to laminate such a crystal layer as Al.sub.x (In.sub.y Ga.sub.(1-y)).sub.(1-x) P (0&lt;x&lt;, 0&lt;y&lt;1) or In.sub.x Ga.sub.(1-x) As.sub.y P.sub.(1-y) (0&lt;x&lt;l, 0&lt;y&lt;1) by selecting an appropriate composition range so as to match its lattice constant to that of GaAs.
Substrates whose plane azimuth is a {100} plane or its equivalent are generally used in various electron devices, such as the above-described field effect transistors and semiconductor lasers. However, in vapor phase epitaxy by MOCVD as illustrative above, a so-called off-azimuth substrate, which does not have an accurate {100} plane but a plane whose normal is slightly slanted from a &lt;100&gt; direction, is generally used.
There are several reasons for the use of an off azimuth substrate. For example, Uchida, et al. mention that the surface defect density and uniformity of a crystal layer can be improved by slanting a {100} plane to one of the &lt;100&gt; directions included in the {100} plane at an angle of 1.degree. to 6.degree. (See JP-B-4-65037). Maeda et al. describe that electron mobility can be improved by slanting a {100} plane to either direction at an angle of 3.degree. to 9.degree. in the preparation of a crystal for HEMT (see JP-A-3283427, the term "JP-A" as used herein means an "unexamined published Japanese patent application").
Thus, the effects of using an off-azimuth substrate widely covers improvements in crystal surface condition, uniformity, and crystal properties. To account for the mechanism of action of an off-azimuth substrate, it is considered that introduction of a slight slant results in formation of a periodical step structure on a terrace of a {100} plane, and each step proceeds in order at the crystal growth (step flow mode) to produce favorable effects on crystal properties.
Anyhow, with many merits as mentioned above, use of an off azimuth substrate in vapor phase epitaxy is regarded as one of fundamental techniques. Epitaxial substrates having a plane azimuth slanted from a &lt;100&gt; direction at an angle of from about 1.degree. to 9.degree. have been widely used on an industrial scale.
As previously stated, epitaxial growth has generally been conducted under such conditions that the lattice constant of an epitaxial growth layer may agree with that of a substrate used, i.e., lattice matching conditions. Accordingly, in using a GaAs substrate, Al.sub.x (In.sub.y Ga.sub.(1-y)).sub.(1-x) P (0&lt;x&lt;1, 0&lt;y&lt;1) and In.sub.x Ga.sub.(1-x) As.sub.y P.sub.(1-y) (0&lt;x&lt;, 0&lt;y&lt;1) having a specific composition as well as AlGaAs have been used as a main material for constituting an epitaxial growth layer.
Although In.sub.x Ga.sub.(1-x) As (0&lt;x&lt;1) has the qualities of a hetero-junction material because of its excellent electron transporting characteristics and possibility of greatly varying the energy gap thereof depending on the composition, it is incapable of matching its lattice constant to that of GaAs. It has therefore been impossible to form an In.sub.x Ga.sub.(1-x) As epitaxial growth layer on a GaAs substrate to provide a semiconductor epitaxial substrate having sufficient properties. Hence, GaAs as a substrate is often replaced with InP. For example, it has been studied to use an InP substrate whose lattice constant agrees with that of In.sub.x Ga.sub.(1-x) As wherein x is about 0.49.
However, the latest technological advancement has revealed that a reliable hetero-junction can be obtained even from a lattice mismatching system without causing reductions in crystal properties, such as dislocation, as far as the lattice mismatching is within the elastic deformation limit. The elastic deformation limit is given as functions of composition and layer thickness. For example, it is known that the following equation (1) theoretically applies for the elastic deformation limit of InGaAs for a GaAs substrate (see Mathews, et al., J. Crystal Growth, Vol. 27, p. 118 (1974) and ibid, Vol. 32, p. 265 (1976)). This equation has recently been proved practically correct through experimentation. ##EQU1## wherein Lc represents a critical layer thickness; a represents a lattice constant of GaAs; .sigma. represents Poisson ratio; and x represents an In content.
Use of such a strain layer within a specific composition and thickness range has thus made it possible to produce an epitaxial substrate partly comprising an InGaAs layer on a GaAs substrate. For example, a layer of In.sub.x Ga.sub.(1-x) As wherein x is 0.15 having a thickness of about 15 nm can be prepared under usual crystal growth conditions without reducing the crystal properties. Such an In.sub.x Ga.sub.(1-x) As layer can be interposed between a GaAs buffer layer and an n-type AlGaAs electron-donating layer to prepare an epitaxial substrate which provides HEMT having more excellent noise characteristics than conventional HEMT comprising GaAs and AlGaAs. Additionally, such a thin layer of In.sub.x Ga.sub.(1-x) As can be utilized as an active layer on a GaAs substrate to provide a semiconductor laser having an emission wavelength band of from 900 to 1000 nm, which has not heretofore been attained in using a GaAs substrate.
In the preparation of the above-mentioned epitaxial substrate partly having a strain layer of InGaAs on a GaAs substrate, precise control of crystal growth on the order of several tens of nm is demanded. Therefore, MBE or MOCVD has been exclusively adopted as a means for crystal growth on account of their excellent control performance. However, MBE and MOCVD involve problems in terms of industrial productivity or quality of devices.
More specifically, although MBE is very excellent in controlling thin films, the crystal obtained by MBE involves many surface defects, which leads to a considerable reject rate, and also MBE has a low crystal growth rate and requires ultra-high vacuum, thus having production problems. MOCVD, on the other hand, produces a crystal layer with excellent surface conditions and achieves high productivity as having been conventionally used for general GaAs lattice matching systems. However, devices using an epitaxial substrate obtained by MOCVD do not always exhibit satisfactory characteristics as compared with those using an epitaxial substrate prepared by MBE with the design being equal.
For example, where an HEMT using a 15 nm thick In.sub.x Ga.sub.(1-x) As (x=0.15) as a channel layer is prepared by using a epitaxial substrate obtained by MOCVD, and radio-frequency characteristics are measured, the noise index at 12 GHz is found to be 0.8 to 0.9 dB. This value is about 0.1 to 0.2 dB greater than that of a device of the same design but using an epitaxial substrate prepared by MBE. As a result of parameter analysis, the main cause of the higher noise index is attributed to transconductance of HEMT. That is, the transconductance of HEMT using a crystal substrate obtained by MOCVD was found to be lower than that of HEMT using a crystal substrate obtained by MBE by about 5 to 15%.
An epitaxial substrate obtained by MOCVD is inferior in device characteristics, whereas that obtained by MBE has poor surface conditions and poor productivity. Under these circumstances, it has been keenly demanded to develop an epitaxial substrate comprising InGaAs on a GaAs substrate which exhibits excellent characteristics and has satisfactory surface conditions for ensuring a satisfactory yield of devices and can be supplied stably on an industrial scale.